The present invention relates to electrostatic control elements. More specifically, the present invention relates to a mirror transducer for controlling the path length of a ring laser gyro.
Ring laser gyroscopes require some means of path length control. As is well known in the art path length control is used to maintain the high intensity resonant signal which propagates within the laser cavity. By maintaining the resonance of the laser at the peak of the gain curve, the performance of the ring laser gyroscope is vastly improved.
Path length control has been achieved by a number of transducer assemblies. Typically these transducer assemblies are driven by one or more piezoelectric elements. Examples of these piezoelectric transducer assemblies, as used in ring laser gyros, can be seen in U.S. Pat. No. 3,581,227 issued to Podgorski, U.S. Pat. No. 4,915,492 issued to Toth, and U.S. Pat. No. 5,148,076 issued to Albers, et al, all of which are assigned to the assignee of the present invention.
In a ring laser gyroscope a gas discharge laser is created within a polygonal closed loop path which in turn causes two counter propagating light beams to exist within this closed loop path. To improve gyro performance it is necessary to adjust the path length of the closed loop path to allow the optical signals within this closed loop path to resonate at a maximum intensity. Secondly, it is necessary to adjust the path length to account for undesired changes in path length. Expansion or contraction due to changes in temperature is one source of undesired path length change.
A wide variety of devices have been developed to accommodate this adjustment in laser path length. As illustrated in U.S. Pat. No. 3,581,227 issued to Podgorski, a transducer element can be used to adjust the position of a mirror within the laser gyro cavity. As shown in FIG. 1 and described in the Podgorski patent, path length control can be realized by a thermally stable transducer block having a cylindrical center post 5, an integral diaphragm 6 extending radially outward from the center post 5 and an annular outer member 4 which is also integral with the diaphragm 6. A mirror 7 is mounted on one end of the central post 5. The transducer is attached to the ring laser gyro block 40 in a gas tight manner. Positioned directly behind and axially aligned with the central post 5 is a stack of piezoelectric ceramic elements 1. The piezoelectric ceramic elements 1 are used to drive or move the mirror 7 causing adjustment of its position. Mounted behind the stack of piezoelectric ceramic elements 1 is a rigid backing 2. This backing 2 must provide support when the piezoelectric elements expand, thus directing all of their force towards the transducer central post 5. This backing 2 requires fairly rigid epoxy bond and a fairly rigid element to provide the necessary support.
All of the previously mentioned transducers utilize and expand on the principals illustrated in the '227 patent. One particular elaboration is the creation of a dual diaphragm transducer element wherein the central post is connected to the annular outer member by two thin film diaphragm elements. Again, the diaphragm elements are integral with and extend radially outward from the cylindrical center post element. As with the Podgorski transducer, these double diaphragm transducer elements utilize piezoelectric elements to create the force necessary to cause movement of the cylindrical center post.
Ring laser gyroscopes are required to operate over a wide range of temperatures. Because of this temperature requirement thermal expansion and contraction can create many problems including the previously mention changes in laser path length. One purpose of the path length control transducer (PLC transducer) is to adjust the laser path length for these expansions and contractions.
The transducers of the prior art have a limited range of movement. Therefore, complex mode reset circuitry is necessary to maintain the transducer within its operating range. Mode reset involve moving the transducer mirror from one resonant peak to another. Since these mode resets detrimentally affect the performance of the gyro, it is desired that the number of mode resets be reduced. Thermal expansion is a source of mode resets, therefore it is desired that thermal expansion be controlled as much as possible, thus reducing the number of mode resets.
Numerous methods have been attempted to control the thermal expansion of the transducer element. One example of such method is the Albers et al. patent which discloses the use of both electrodes and piezoelectric ceramic to provide a means for moving the transducer mirror. However, it is still difficult to control the thermal expansion characteristics of piezoelectric ceramics.